1. Technical Field of the Invention
The present invention relates to a switch used for controlling switching-on and-off of a stop lamp that is turned on and off in operable connection with a brake pedal in an automobile.
2. Description of Related Art
There are two types of stop lamp switch of an automobile: One is a large-current type, wherein a stop lamp circuit connected to a power source is directly opened and closed with a switch, and the other is a small-current type, wherein such stop lamp circuit is indirectly opened and closed by the control of a microprocessor in an electronic circuit through relays or the like. With the progress of overall electronic control of automobiles in recent years, the latter has found more applications.
One of the conventional switches will be described below with reference to FIGS. 11 to 14.
FIG. 11 is a cross sectional view of a conventional switch and FIG. 12 is an exploded perspective of the same. The switch 10 comprises a cylindrical case 1 made of insulating resin having an inner bottom surface 1A and an upper open end. A pair of stationary contacts 2 consisting of a conductive metal plate is embedded in the inner wall 1B of the case 1. The distal ends of this pair of stationary contacts 2 make connection parts 2A and protrude through apertures in the bottom of the case 1, these being connected to an electronic circuit of the automobile through a connector (not shown).
Reference numeral 3 denotes a contact piece that is made of a conductive thin metal plate having resiliency, and numeral 4 denotes an operation shaft that is accommodated within the case 1 such as to be displaceable in an upward and a downward directions. One end of the contact piece 3 is fixed to one side (right hand side in the drawing) of a support portion 4A at the lower end of the operation shaft 4 with a rivet 5, while the other end, which makes a pair of movable contacts 3A, is resiliently pressed against the stationary contacts 2. The contact piece 3 has two arms 3B connecting the movable contacts 3A with the base end of the contact piece, and these are slightly deformed when the movable contacts 3A are in contact with the stationary contacts 2. A coil spring 6 is inserted in a compressed state in a cavity 4B formed on the underside of the operation shaft 4 and pressed against the inner bottom surface 1A of the case 1 so that it gives the operation shaft 4 an upward biasing force.
The upper open end of the case 1 is covered by a cover 7, which has a through-hole 7A in the center thereof. A cylindrical bearing 8 having threads on the outer periphery thereof is fixed on the through-hole 7A, so that the operation shaft 4 extends through the through-hole 7A of the cover 7 and the hole 8A bored in the bearing 8.
The switch 10 constructed as described above is mounted to an automobile and operates as described below.
FIG. 13 is a side view showing a state wherein the switch 10 is mounted on an automobile. Reference numeral 11 denotes a brake pedal, which is integrally formed with an angle 11A. The upper end of the angle 11A is supported around a support 11B such as to be rockable in relation to the automobile body. The switch 10 is mounted to the automobile body with nuts 12 which engage the threads on the bearing 8 in a state wherein the operation shaft 4 is pressed by the angle 11A.
As previously mentioned, a connector 13 is attached to the switch 10, so that one of the pair of stationary contacts 2 is directly connected to an electronic circuit 15 of the automobile, while the other stationary contact 2 is connected to the electronic circuit 15 through a power source 14. The electronic circuit 15 comprises microprocessors for controlling a stop lamp circuit, anti-lock brake system, traction control device, and others.
While the operation shaft 4 of the switch is pressed by the angle 11A, the movable contacts 3A of the contact piece 3 are resiliently pressed against the inner wall 1B of the case 1 at a point that is away from the stationary contacts 2, as shown in FIG. 14A. When the brake pedal 11 is depressed, as indicated by broken lines in FIG. 13, the angle 11A rocks around the support 11B as the fulcrum. When the angle 11A departs from the operation shaft 4 of the switch 10, the operation shaft 4 returns to the state shown in FIG. 11 by the force of the coil spring 6, whereupon the movable contacts 3A of the contact piece 3 slide against the inner wall 1B of the case 1 as being resiliently pressed thereto, and contact the pair of stationary contacts 2 as shown in FIG. 14B. The two stationary contacts 2 are thus connected to each other through the contact piece 3, thereby turning on the stop lamp.
In such switch as described above, because the movable contacts 3A of the contact piece 3 make point contact with the stationary contacts 2 and they slide against the stationary contacts 2 and the inner wall 1B of the case 1 always at the same point, when foreign substances exist between the contacting parts, they cannot establish reliable contact with each other. Particularly, it is often the case that foreign substances, such as galls of the insulating resin of the case 1 or carbides due to the arcs generated when the contact piece 3 contacts with and separates from the stationary contacts 2, are deposited on the stationary contacts 2.
Moreover, the contact piece 3 is held on one side of the support part 4A at the lower end of the operation shaft 4, and the movable contacts 3A of the contact piece 3 are pressed against the stationary contacts 2 through the arms 3B that are slightly deformed. A certain load is thus exerted to the operation shaft 4 on one side, whereby the operation shaft 4 is accommodated in a slightly tilted manner within the case 1 and the bearing 8, within a range determined by the gaps present between the operation shaft 4 and the hole 8A of the bearing 8. Consequently, when mounted on the automobile, as the brake pedal 11 is depressed and the angle 11A departs from the operation shaft 4, whereby the operation shaft 4 is released from the pressing force from the angle 11A and restores to its initial position by means of the coil spring 6, the switch 10 may sometimes produce noises, as the tilted operation shaft 4 collides against the case 1 or the bearing 8.